1. Field of the Invention
The invention relates to electrolytic capacitors comprising specifically substituted poly(alkylenedioxythiophenes) as solid electrolytes and their production, and also relates to conductive layers of specifically substituted poly(alkylenedioxythiophenes), their preparation and use.
2. Brief Description of the Prior Art
The class of π-conjugated polymers (also referred to as conductive polymers or synthetic metals) has been the subject of numerous publications in recent decades. Such conductive polymers are gaining increasing economic importance, since they have advantages over metals with respect to processability, weight and the ability to adjust properties in a targeted manner by means of chemical modification.
Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes). Their method of preparation and use, as well as the associated disadvantages are described below.
A review of numerous poly(alkylenedioxythiophene) derivatives, in particular poly-(3,4-ethylenedioxythiophene) derivatives, their monomer building blocks, syntheses and uses are given by L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12 (2000) 481–494. A particularly important and industrially utilized polythiophene is poly-3,4-(ethylene-1,2-dioxy)thiophene, often also referred to as poly(3,4-ethylenedioxythiophene), which in its oxidized form displays very high conductivities and is described, for example, in EP-A 339 340. U.S. Pat. No. 5,111,327 and U.S. Pat. No. 5,187,608 describe the use of substituted poly-(3,4-alkylenedioxythiophenes) as electroactive polymers, e.g. in electrochromic windows (smart windows). Blohm et al. (U.S. Pat. No. 5,111,327 and U.S. Pat. No. 5,187,608) have shown that the conductivity of polymer layers of substituted 3,4-alkylenedioxythiophenes prepared by chemical oxidation is higher after subsequent electrochemical reduction followed by electrochemical oxidation than that of corresponding layers of unsubstituted poly(3,4-ethylenedioxythiophene). However, the electrochemical reduction and reoxidation introduces a considerable complication into the process.
The European patent specification EP-A 340 512 describes the preparation of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of its cationic polymer prepared by oxidative polymerization as solid electrolyte in electrolytic capacitors. Poly(3,4-ethylenedioxythiophene) as replacement for manganese dioxide or charge transfer complexes in solid electrolyte capacitors reduces the equivalent series resistance of the capacitor and improves the frequency behaviour as a result of the higher electrical conductivity.
Leakage current of such a capacitor depends essentially on the quality of the polymer film: if graphite or silver penetrates through the polymer film and thus comes into contact with the dielectric, the leakage current increases drastically since defects in the oxide layer can no longer be isolated via the local destruction of the conductive polymer (self-healing effect).
In their preparation, after a chemical polymerization, it may be necessary to wash out the salts, i.e. excess oxidant and its reduced form, in order to obtain layers of satisfactory quality. Otherwise, crystallization of the salts can lead to an increased series resistance over the course of time due to formation of contact resistances. In addition, the crystals can damage the dielectric or the outer contact layers when the capacitor is mechanically stressed, so that the leakage current rises. It is therefore desirable to suppress the crystallization of salts of the oxidant or residual salts of its reduced form which remain in the capacitor despite washing.
There is therefore a continuing need to increase the conductivity and related quality of known layers of poly(3,4-ethylenedioxythiophene), particularly in respect of the above-described use in electrolytic capacitors, in order to achieve improved performance. More particularly, it is desirable to achieve a further decrease in the equivalent series resistance and the leakage current of solid electrolyte capacitors. In addition, simple methods of production of the layers or the electrolytic capacitors are desirable.
It is therefore an object of the invention to provide or discover suitable electrically conductive polymers which can be used for preparation of electrically conductive layers and especially as solid electrolytes in electrolytic capacitors, and to improve the conductivity and related quality, in particular better binding of residual salts and improved homogeneity, as compared to the properties of known polymers such as poly(3,4-ethylenedioxythiophene).